Cylindrically machining apparatus

ABSTRACT

A machining apparatus for improving the roundness of a work. The outer circumference of a work is divided into predetermined intervals, and a difference between the radius of that portion of the work after machined and a target radius thereof is detected for each of the intervals. The position of a tool which corresponds to every predetermined rotation angle is corrected on the basis of a detected radius difference at each of the intervals. The path of the tool is interpolation-computed on the basis of the corrected positions. The tool is turned along the interpolated path, so that the work is machined with a high precision.

TECHNICAL FIELD

The present invention relates to an apparatus, such as a crankshaftmiller, for machining a work into a product having a cylindricalcircumference, and more particularly to a cylindrically machiningapparatus capable of improving roundness in the circumference of amachined product.

BACKGROUND ART

A crankshaft miller of a type in which a milling cutter with the innerteeth being contacted with the outer surface of a work rotates at a highspeed, thereby machining the journal of a crankshaft to have acylindrical circumference, is known. A motion of the milling cutter isshown in FIGS. 14(a) to 14(d) in which the center c of the millingcutter 1 moves in the direction of an arrow d along a path (circle) blocated at a predetermined distance r from the center a of a work WK. Aplurality of tips 1a formed on the inner side of the milling cutter 1rotates about the center c, thereby cutting the outer surface of thework.

The milling cutter 1 is moved under the control of a program prepared byan NC machine. In the NC program, the motion of the milling cutter isdefined by a single command given by, for example,

G02 X90. Y75. I0. J35.

In the command, "G02" indicates "arc interpolation designation", "X90.Y75." indicates "X and Y coordinates at the start point S and the endpoint E (same as the start point S) of the center c of the millingcutter 1" and "I0. J35." indicates "X and Y coordinates of therotational center a relative to the start point S".

In machining the outer surface of the work WK such as a crankshaft,rigidity of the work varies with the position of the rotation.Accordingly, roundness of the circumference of the machined work WKvaries as indicated by a contour C' in FIG. 15, which is deformed from acircle C, causing an error. To make the contour C' of the work WKcoincident with the circle C by displacing inward or outward thesegments e, f, g, and h of the contour C', the path through which thecenter of the milling cutter 1 moves must be corrected into a path b'having segments e', f', g', and h' as shown in FIG. 16 according to thedeformation of the segments e, f, g, and h.

As mentioned above, commands used in an NC machine are such a commandthat "move along a circle of a fixed radius". The correction that iscarried out by the command is limited to a correction of the millingcutter motion over the entire circle. It is impossible to correct eachsegment of the contour indicated by the path b'. With such a command, itis impossible to improve roundness of the contour of a machined work.

The present invention has been made in view of the above background andits object is to provide a machining apparatus in which a path of itscutting tool is corrected according to the commands different for eachsegment of the circumference of a work, thereby remarkably improvingroundness of the resultant product.

DISCLOSURE OF INVENTION

To achieve the above object, there is provided a cylindrically machiningapparatus for machining a work by turning a tool along a circumferenceof the work so that the circumference has a target radius, comprisingdetection means for dividing the circumference of the work intopredetermined intervals and for detecting a difference between radius ofthe work after machined and a target radius thereof for each of theintervals; calculating means for correcting the position of the toolevery predetermined rotation angle based on the radius differencedetected by the detecting means, and for interpolation-computing a pathof the tool based on the corrected position of each rotation angle; andcontrol means for turning the tool along the path calculated by thecalculating means.

With such a construction of the invention, the circumference of a workis divided into predetermined intervals and difference between theradius of a work after machined and the target radius for each of theintervals is detected. The position of the tool is corrected at everypredetermined rotation angle according to the detected radiusdifference. A path of the tool is interpolation-computed according tothe corrected position of each rotation angle and the tool is turnedalong the interpolation-computed path. Thus, a work is machined with ahigh precision.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing a sequence of steps of a machiningprocedure in an embodiment of a cylindrically machining apparatusaccording to the present invention;

FIG. 2 is a flowchart showing a sequence of steps for preparing amachining program in FIG. 1;

FIG. 3 is a flowchart showing a sequence of steps for preparing acorrection machining program shown in FIG. 1;

FIG. 4 is a diagram showing the relative positions of the portions of acrankshaft miller applied to the embodiment;

FIG. 5 is a diagram showing the segmented path of the center position ofa milling cutter;

FIG. 6 is a diagram showing a relationship between the center of a workand the center positions of a milling cutter;

FIG. 7 is a diagram showing the path of the center position of a millingcutter after it is corrected;

FIG. 8 is a diagram showing a relationship between a machining error ofa work and a quantity of correction of the path of the center of amilling cutter;

FIG. 9 is a diagram for explaining an example of correcting calculationin the embodiment;

FIG. 10 is a diagram for explaining another example of correctingcalculation in the embodiment;

FIG. 11 is a diagram for explaining still another example of correctingcalculation in the embodiment;

FIG. 12 is a diagram for explaining yet another example of correctingcalculation in the embodiment;

FIG. 13 is a diagram for explaining a further example of correctingcalculation in the embodiment;

FIGS. 14(a) through 14(d) show a series of diagrams for explaining amotion of a milling cutter in a crankshaft miller;

FIG. 15 is a diagram showing the result of measuring roundness of a workmachined by a milling cutter; and

FIG. 16 is a diagram showing a path of the center of a milling cutterthat is to be corrected according to the measuring result of FIG. 15.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of a cylindrically machining apparatus according to thepresent invention will be described with reference to the accompanyingdrawings. In the embodiment, the present invention is applied for acrankshaft miller. The motion of a milling cutter in the crankshaftmiller is similar to that already described referring to FIG. 14. Samereference characters used in FIG. 14 will be used also in thedescription of the embodiment.

Referring to FIG. 1 showing a sequence of steps of a machiningprocedure, a machining program is prepared using known data in an NCmachine (step 101). In this step, a routine shown in FIG. 2 is carriedout. As shown in FIG. 4, a machining radius P of a work WK, i.e., thejournal WK of a crankshaft, and the inner radius g of a milling cutter 1as a cutting tool are known. Using these radiuses, a radius r of a pathfor the center C of the milling cutter 1 is calculated (step 201). Asshown in FIG. 5, a path b of the cutter center is divided, every 15°,into 24 segments . Then, X-Y coordinates (x1, y1), (x2, y2),. . . (x0,y0) at the points (1), (2), . . . (24) ((0)) on the divided path b arecalculated (step 202).

Subsequently, an NC program to move the center c of the milling cutter 1along a predetermined path is prepared. Since the center c of the cutter1 moves along the circumference of the work, each segment forms an arc.The radius r and the center a of each arc are designated.

An NC program for designating the radius of the arc is prepared forevery segment of the path of the cutter center as follows:

    ______________________________________                                                   G02 Xx1 Yy1 Rr                                                                G02 Xx2 Yy2 Rr                                                                G02 Xx3 Yy3 Rr                                                                . . .  . . .  . . .  .                                             ______________________________________                                    

The program is shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Point  NC program     Point  NC program                                       ______________________________________                                        (1)    G02Xx1Yy1Rr    (13)   G02Xx13Yy13Rr                                    (2)    G02Xx2Yy2Rr    (14)   G02Xx14Yy14Rr                                    (3)    G02Xx3Yy3Rr    (15)   .                                                (4)    G02Xx4Yy4Rr    (16)   .                                                (5)    .              (17)   .                                                (6)    .              (18)   .                                                (7)    .              (19)   .                                                (8)    .              (20)   .                                                (9)    .              (21)   .                                                (10)   .              (22)   .                                                (11)   .              (23)   .                                                (12)   .              (24)   G02Xx0Yy0Rr                                      ______________________________________                                    

In the program, "Xx1 Yy1 Rr" means "Move along the arc of the radius rof a segment from a start point (24) or (0) to a point (1)". "Xx2 Yy2Rr" means "Move on the arc of the radius r of a segment from a point (1)to a point (2)". The remaining commands have similar meanings (step203).

In preparing the program, the center position a of the arc may be usedfor the instruction of the motion of the milling cutter, instead of theradius of curvature r that is used in the program stated above. Whenusing the center position a, the center position a of the arc isdesignated at every division point. The center position a of the arc isspecified as a position (in, jn) relative to the center position a asthe start point of the arc. For an arc ranging from a division point (2)to another division point (3) as shown in FIG. 6, the start point is thepoint (2) and its coordinates are (x2, y2). The center position a isspecified by the distances i2 and j2 of the coordinates (x2, y2)relative to a point a. Polarities are assigned to the relative distancesi2 and y2 according to the X-axis and Y-axis directions. Thus, an NCprogram for designating the center position a of the arc is prepared forevery segment as follows:

    ______________________________________                                        G02 Xx1Yy1Ii0Jj0                                                              G02 Xx2Yy2Ii1Jj1                                                              G02 Xx3Yy3Ii2Jj2                                                              . . .  . . .  . . .                                                           ______________________________________                                    

In the program, "G02 Xx1Yy1Ii0Jj0" means "in the segment from a startpoint (24) or (0) to a division point (1), the center of the arc ispositioned at a distance i0 from the start point (24) in the X-directionand at a distance j0 in the Y-axis". The remaining commands mean similarinstructions (step 205).

After the NC program is prepared, the procedure returns to the step 101in FIG. 1. Under the numerical control according to the prepared NCprogram, the milling cutter 1 machines the work while the center c ofthe milling cutter 1 depicts the path b as shown in FIG. 5 (step 102).

Roundness of the machined work WK is measured (step 103). Then, acorrection machining program is prepared on the basis of the measuringresult (step 104). The procedure goes to a routine shown in FIG. 3. Inthe routine, the outer surface of a work WK is divided everypredetermined angle 8 (15°) into segments as in the step 202 (step 301).Difference between the target radius of the work WK, namely, a distancefrom the center a of the work WK to the circumference thereof when thecircumference of the work is a circle C and distances from the center aof the work WK after it is machined to the circumference C' in otherwords differences Δh0, Δh1, . . . are measured at every predeterminedangle θ (15°), e.g., at the division points (0), (1), . . . , as shownin FIG. 8 (step 302).

The differences Δh0, Δh1, . . . are added to the coordinates (x0, y0),(x1, y1), . . . at the division points (0), (1), . . . As a result, thecoordinates at the division points (0), (1), . . . are corrected asfollows:

    ______________________________________                                        x'0 = x0                                                                      y'0 = y0 - Δh0                                                          x'1 = x1 - Δh1 · sinθ (where x1 = -rsinθ)          y'1 = y1 - Δh1 · cosθ (where y1 = -rcosθ)          . . .                                                                         x'n = xn - Δhn · sinθ                                    y'n = yn - Δhn · cosθ                                                         . . .         (1)                                        ______________________________________                                    

When the difference of the work WK at the position 0° is Δh0 where C' islarger than C, the center of the milling cutter 1 is shifted Δh0 outwardin order to displace Δh0 the circumference of the work WK inward at thecorresponding division point (0). When the difference of the work WK atthe position 0°+θ is Δh1 where C' is larger than C, the center of themilling cutter 1 is shifted Δh1 outward in order to displace Δh1 thecircumference of the work WK inward at the corresponding division point(1). At other division points (2), (3), etc., the center of the cutter 1is shifted in like manner (step 303).

Subsequently, each segment between the adjacent division points areinterpolated by an arc according to the corrected coordinates (xn, yn)(n=0, 1, . . .). A radius r'n (n=1, 2, . . .) of each arc is calculated(FIG. 7) or the center a' of each arc is calculated. The calculation isperformed in various ways as given hereinafter (steps 304 and 306).

First calculation (FIG. 9)

In a first calculation, a middle point between the adjacent divisionpoints is calculated, and a radius r'n is calculated using these threepoints, as shown in FIG. 9. In an example of the segment between thedivision points (0) and (1), the cutter center position (x"1, y"1)corresponding to the intermediate angle 0°+θ/2 between the positions 0°and 0°+θ (FIG. 8) on the circumference of the work WK is generated asgiven by the following equation (2).

    x"1=-r·sinθ/2-{(Δh0+Δh1)/2}sinθ/2

    y"1=-r·cosθ/2-{(Δh0+Δh1)/2}cosθ/2(2)

The equation holds on the assumption that a difference at the angle0°+θ/2 is a mean value of the difference Δh0 at 0° C. and the differenceΔh1 at θ. The radius r'1 of the arc passing these three points (x'0,y'0), (x"1, y"1), (x'1, y'1) is calculating by the followingsimultaneous equations. The result of the calculation is the radius r'1of the motion of the cutter center.

    (x'0-X).sup.2 +(y'0-Y).sup.2 =(r'1).sup.2

    (x"1-X).sup.2 +(y"1-Y).sup.2 =(r'1).sup.2

    (x'1-X).sup.2 +(y'1-Y).sup.2 =(r'1).sup.2                  (3)

where

X: coordinate of the center of the arc in the X-direction

Y: coordinate of the center of the arc in the Y-direction

A relative distance (i'0, j'0) is calculated as shown in FIG. 6 toobtain

    i'0=X-x'0

    j'0=Y-y'0                                                  (4)

Thus, the center a' of the rotation of the milling cutter 1 (the centerin the interpolated arc) is specified. Subsequently, the radius r'n forn=2, 3, . . . and the relative distance (i'n, j'n) for n=1, 2, . . . arecalculated as in the manner of the equations (3) and (4).

Second Calculation (FIG. 10)

In the second calculation, the radius r'n and the relative distance(i'n, j'n) are calculated on the basis of the corrected coordinates atthree successive division points.

In an example of three division points (0), (1), and (2) shown in FIG.10, the radius r'1 (r=r'2) of an arc passing through the correspondingthree points (x'0, y'0), (x'1, y'l), and (x'2, y'2) are calculated usingthe following simultaneous equations. The resultant solution is used asthe radius r'1 of motion of the cutter center.

    (x'0-X).sup.2 +(y'0-Y).sup.2 =(r'1).sup.2

    (x'1-X).sup.2 +(y'1-Y).sup.2 =(r'1).sup.2

    (x'2-X).sup.2 +(y'2-Y).sup.2 =(r'1).sup.2                  (5)

where

X: coordinate of the center of the arc in the X-direction

Y: coordinate of the center of the arc in the Y-direction

The relative distances (i'0, j'0) and (i'1, j'1) are calculated asfollows.

    i'0=X-x'0

    j'0=Y-y'0

    i'0=X-x'1

    j'0=Y-y'1.                                                 (6)

Thus, the center a' of the rotation of the cutter 1 is specified.Subsequently, the radius r'n for n=2, 3, . . . and the relative distance(i'n, j'n) for n=1, 2, . . . are calculated as in the manner of theequations (5) and (6).

Third calculation (FIGS. 12 and 13)

In this calculation, the radius r'n is obtained by adding a radiusdifference to the coordinates at each division point. In an example ofthe division point (0) shown in FIG. 12, the radius r'1 of the arcbetween the division points (0) and (1) is calculated by

    r'1=r+Δh0                                            (7)

The coordinates (X, Y) of the center a' of the arc is obtained bysolving the following simultaneous equations

    (x'0-X).sup.2 +(y'0-Y).sup.2 =(r1).sup.2

    (x'1-X).sup.2 +(y'1-Y).sup.2 =(r1).sup.2                   (8)

The relative distance (i'0, j'0) is calculated as follows:

    j'0=X-x'0

    j'0=Y-y'0                                                  (9)

Thus, the center a' of the rotation of the cutter 1 is specified.Subsequently, the radius r'n for n=2, 3, . . . and the relative distance(i'n, j'n) for n=1, 2, . . . are calculated as in the manner of theequations (7) and (9).

Fourth calculation

In the calculation, correction is made for the division points but theradius E initially set is used as it is for the radius r. In an exampleof the arc between the division points (0) and (1), the coordinates (X,Y) of the center a' of the arc is obtained by solving the followingsimultaneous equations

    (x'0-X).sup.2 +(y'0-Y).sup.2 =r.sup.2

    (x'1-X).sup.2 +(y'1-Y).sup.2 =r.sup.2                      (10)

Accordingly, the relative distance (i'0, j'0) is

    j'0=X-x'0

    j'0=Y-y'0                                                  (11)

Thus, the center a' of the rotation of the cutter 1 is specified.Subsequently, the radius r'n for n=2, 3, . . . and the relative distance(i'n, j'n) for n=1, 2, . . . are calculated as in like manner.

Fifth calculation (FIG. 11)

In this calculation, a distance between the coordinates of the divisionpoint after corrected and the center position a of the work center isused for the radius r'n. In an example of the arc between the divisionpoints (0) and (1), the radius r'1 is calculated using the followingequation:

    r'1={(x'1-X0).sup.2 +(y'-Y0).sup.2 }.sup.1/2               (12)

where (X0, Y0) is the coordinates of the work center. This issubstantially the same as that in the third calculation. Accordingly, asin FIG. 13, the coordinates (X, Y) of the center a' of the arc isobtained by solving the following simultaneous equations:

    (x'0-X).sup.2 +(y0-Y).sup.2 =(r'1).sup.2

    (x'1-X).sup.2 +(y1-Y).sup.2 =(r'1).sup.2                   (13)

The relative distance (i'0, j'0) is calculated as follows:

    j'0=X-x'0

    j'0=Y-y'0                                                  (14)

Thus, the center a' of the rotation of the cutter 1 is specified.Subsequently, the radius r'n for n=2, 3, . . . and the relative distance(i'n, j'n) for n=1, 2, . . . are calculated as in like manner.

Using the radius r'n thus calculated, the NC program for each segment isprepared for every segment as follows:

    ______________________________________                                        G02 Xx'1Yy'1Rr'1                                                              G02 Xx'2Yy'2Rr'2                                                              G02 Xx'3Yy'3Rr'3                                                              . . .                                                                         ______________________________________                                    

The NC programs prepared are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Point  NC program     Point  NC program                                       ______________________________________                                        (1)    G02Xx'1Yy'1Rr'1                                                                              (13)   G02Xx'13Yy'13Rr'13                               (2)    G02Xx'2Yy'2Rr'2                                                                              (14)   G02Xx'14Yy'14Rr'14                               (3)    G02Xx'3Yy'3Rr'3                                                                              (15)   .                                                (4)    .              (16)   .                                                (5)    .              (17)   .                                                (6)    .              (18)   .                                                (7)    .              (19)   .                                                (8)    .              (20)   .                                                (9)    .              (21)   .                                                (10)   .              (22)   .                                                (11)   .              (23)   .                                                (12)   .              (24)   G02Xx'24Yy'24Rr'24                               ______________________________________                                    

In the above program, "Xx'1 Yy'1 Rr'1" means "Move along the arc of theradius r'1 of curvature in the segment from a start point (24) or (0) toa point (1)". "Xx'2 Yy'2 Rr'2" means "Move along the arc of the radiusr'2 of curvature in the segment from a point (1) to a point (2)". Theremaining commands have similar meanings (step 305). Thus, an NC programfor designating the center position of the arc, i.e., the relativedistance (i'n, j'n), is prepared for every segment as follows:

    ______________________________________                                        G02 Xx'1Yy'1Ii'0Jj'0                                                          G02 Xx'2Yy'2Ii'1Jj'1                                                          G02 Xx'3Yy'3Ii'2Jj'2                                                          . . .                                                                         ______________________________________                                    

In the program, "G02 Xx'1Yy'1Ii'0Jj'0" states "in the segment from astart point (24) or (0) to a division point (1), the center of the arcis positioned at a distance i'0 from the start point (24) in theX-direction and at a distance j'0 in the Y-axis". The remaining commandsstate similar instructions (step 307).

After the NC program is prepared, the procedure returns to the step 105in FIG. 1. Under the numerical control according to the prepared NCprogram, the milling cutter 1 machines the work while the center c ofthe milling cutter 1 depicts the path b' as shown in FIG. 7 (step 105).

In the above-described embodiment the cutter with the inner teeth isused for machining the work since the present invention is applied for acrankshaft miller. The tool may be replaced with any type of tool, suchas a tool with the outer teeth, if it can shape the outer surface of thework as desired.

In the above-described embodiment the path of the cutter center isdivided into 24 segments every 15°. However, the number of the segmentsis not limited to 24 and it may be divided into 36 segments every 10° ,for example.

Further, the tool path may be divided unequally in accordance with thework machining characteristic. For example, it may be divided at thepositions of 0°, 30°, 60°, 80°, 85°, 90°, 120°, etc.

INDUSTRIAL APPLICABILITY

As seen from the foregoing description, the path of the tool is dividedinto a plural number of segments. The path of the tool is corrected forevery segment. Therefore, roundness of the work to be machined isimproved. The resultant accuracy improvement remarkably enhances thereliability of the machine tool.

The improvement of the roundness of a product reduces the number ofsteps of the subsequent process. This leads to a considerable reductionof cost.

We claim:
 1. A cylindrically machining apparatus for machining a work byturning a tool along a circumference of the work so that thecircumference has a target radius, comprising:detecting means fordividing the circumference of the work into predetermined intervals andfor detecting a difference between radius of the work after machined anda target radius thereof for each of the intervals; calculating means forcorrecting the position of the tool at every predetermined rotationangle based on the radius difference detected by the detecting means,and for interpolation-computing a path of the tool based on thecorrected position of each rotation angle; the calculating meansinterpolation computing the path of the tool by calculating a middleposition between adjacent a two corrected positions of the tool, and bydetermining radius of an arc passing through these two correctedpositions and the middle position as radius of the path of the tool; andcontrol means for turning the tool along the path calculated by thecalculating means.
 2. A cylindrically machining apparatus for machininga work by turning a tool along a circumference of the work so that thecircumference has a target radius, comprising:detecting means fordividing the circumference of the work into predetermined intervals andfor detecting a difference between radius of the work after machined anda target radius thereof for each of the intervals; calculating means forcorrecting the position of the tool at every predetermined rotationangle based on the radius difference detected by the detecting means,and for interpolation-computing a path of the tool based on thecorrected position of each rotation angle; the calculating meansinterpolation-computing the path of the tool in such a manner thatradius of an arc passing through adjacent three corrected positions isdetermined as radius of the path of the tool; and control means forturning the tool along the path calculated by the calculating means. 3.A cylindrically machining apparatus for machining a work by turning atool along a circumference of the work so that the circumference has atarget radius, comprising:detecting means for dividing the circumferenceof the work into predetermined intervals and for detecting a differencebetween radius of the work after machined and a target radius thereoffor each of the intervals; calculating means for correcting the positionof the tool at every predetermined rotation angle based on the radiusdifference detected by the detecting means, and forinterpolation-computing a path of the tool based on the correctedposition of each rotation angle; the calculating meansinterpolation-computing the path of the tool in such a manner thatradius of an arc passing through adjacent two corrected positions isequal to the sum of radius of the path of the tool and the differenceassociated with the adjacent corrected positions; and control means forturning the tool along the path calculated by the calculating means. 4.A cylindrically machining apparatus for machining a work by turning atool along a circumference of the work so that the circumference has atarget radius, comprising:detecting means for dividing the circumferenceof the work into predetermined intervals and for detecting a differencebetween radius of the work after machined and a target radius thereoffor each of the intervals; calculating means for correcting the positionof the tool at every predetermined rotation angle based on the radiusdifference detected by the detecting means, and forinterpolation-computing a path of the tool based on the correctedposition of each rotation angle; the calculating meansinterpolation-computing the path of the tool in such a manner thatcenter position of an arc passing through adjacent two correctedpositions is calculated while considering radius of the path of the toolbefore correction as radius of an arc passing through any adjacent twocorrected positions, and the path of the tool is determined to be an arcwhich has the calculated center position as a center thereof and theradius of the path of the tool before correction as a radius thereof andwhich passes through adjacent two corrected positions; and control meansfor turning the tool along the path calculated by the calculating means.5. A cylindrically machining apparatus for machining a work by turning atool along a circumference of the work so that the circumference has atarget radius, comprising:detecting means for dividing the circumferenceof the work into predetermined intervals and for detecting a differencebetween radius of the work after machined and a target radius thereoffor each of the intervals; calculating means for correcting the positionof the tool at every predetermined rotation angle based on the radiusdifference detected by the detecting means, and forinterpolation-computing a path of the tool based on the correctedposition of each rotation angle; the calculating meansinterpolation-computing the path of the tool by determining thatdistance between the center position of the tool and the correctedposition as radius of an arc passing through adjacent two correctedpositions of the tool; and control means for turning the tool along thepath calculated by the calculating means.